I have a legendary (The actual legendaries in the game) team, but I don't know if mewtwo or mew is best for STAB in my team.
All of Mew's stats are base 100 which makes him good for EV training.
Mewtwo is better for a Special Sweeper with a higher SpAtk.
So Mewtwo is better if you're not into EV training.
They both have the same weaknesses although they have different abilities. Movesets are available through the links.
If this didn't answer your question then comment and I'll fix it up.